The evolution of integrated circuit designs has resulted in higher operating frequency, increased numbers of transistors, and physically smaller devices. This continuing trend has generated ever increasing area densities of integrated circuits and electrical connections. To date, this trend has also resulted in both increasing power and increasing heat flux devices, and the trend is expected to continue into the foreseeable future. Further, materials used in electronic packaging typically have various coefficients of thermal expansion. Under temperature fluctuations induced by normal usage, storage, and manufacturing conditions, the various coefficients of thermal expansion may lead to mechanical failures such as material cracking (cohesive failure) and delamination in a region of adjoining materials (adhesive failure). Still further, mechanical failures may be induced by many other causes, e.g. exposure to shock and vibration during shipping to a system or motherboard integrator, system or motherboard assembly, or shock and vibration during delivery to the end customer.
For example, solder bumps often electrically and mechanically couple an integrated circuit die to a package substrate. Further, the package substrate may be electrically and mechanically coupled to a printed circuit board by solder balls. The package substrate may have a coefficient of thermal expansion different from the die and/or the printed circuit board. Under a change in temperature, a mechanical stress may result within the solder balls and solder bumps due to various coefficients of thermal expansion. In some circumstances, the solder balls and solder bumps crack (cohesive failure) under the thermally induced stress. Once a crack initiates, the cohesive failure may propagate at a rate partially dependent on a characteristic dimension of the crack, e.g., diameter at the tip of the crack.
One existing method of preventing solder ball and solder bump cracking includes dispensing a curable material in the regions between the solder balls and solder bumps (“underfilling”). When an underfill is used, some of the stress otherwise taken by the solder balls and solder bumps is taken by the underfill material and thereby reduces the likelihood of solder ball or solder bump cracking. In applications that use presently available technology, if a crack initiates within the underfill, the crack may propagate through the underfill and through the solder ball and solder bump. Often underfill materials are brittle and cracks may propagate readily once initiated. Another existing technology uses underfill materials with increased toughness to slow crack propagation. Some methods of increasing underfill composite toughness include adding a second phase to the cured composite, for example through using any of a variety of rubber additives or loading the composite with particulate inorganic fillers. Though a crack in a brittle underfill may propagate more rapidly than in a toughened material, even a crack in a tough underfill material may still propagate.
In other circumstances, adjoining layers of material within the package may delaminate due to a mechanical stress transferred through the solder balls and solder bumps. Similar to a cohesive failure, an adhesive failure may propagate at a rate partially dependent on a characteristic dimension of the region of delamination. Characteristically poor metal—polymer adhesion exacerbates adhesive failure propagation. One well known method of partially managing delamination failures includes applying an adhesive coating to a material interface. Alternative methods of enhancing adhesive properties of polymer and metal combinations include surface roughening or adding coupling agents, e.g., silyl ethers. Similar to crack propagation, delamination may more readily propagate when an interface coating is brittle than when the interface coating is tough. Likewise, while delamination propagation in a tough interface coating may be slower than in a brittle interface coating, the adhesive failure may still propagate.
Material cracking and delamination may occur under circumstances other than expansion and contraction due to temperature cycling. Circumstances under which cracking and delamination failures may occur are many and include, for example, dynamic warpage of the package during use, fatigue from temperature cycling, and shock and vibration arising through shipping, assembly, and handling.